Macaque monkeys undergoing primary infection with simian hemorrhagic fever (SHF) virus develop an acute, febrile hemorrhagic disease and generally die 5 to 14 days after infected. Occasionally, infected macaques completely recover from infection. Animals infected 6 months or more after recovery with homotypic or heterotypic strains of SHF virus died more rapidly (less than 2 days) than primarily infected animals. At that time of second infection, high serum antibody titers to viral antigens were detected, but this antibody lacked neutralizing activity. Antibody did not appear to be the cause of the more rapid death because macaques receiving passively transferred specific viral antibody did not die any more rapidly than primarily infected animals. These results suggest that vaccination of macaques with SHF virus would afford short lived protection and, in fact, could exacerbate subsequent SHF virus infections. Many of the physical-chemical characteristics of SHF virus, a member of the Togaviridae family, have not been determined. We have found the genome of SHF virus to be a single linear positive stranded molecule of RNA (Mo1. Wt. 5.5 x 10 to the 6 daltons, sedimentation coefficient 49S). SHF virions contain 5 polypeptides ranging in Mo1. Wt. from 50K to 10K daltons.